Genetic organisation of Iris yellow spot virus M RNA: indications for functional homology between the G(C) glycoproteins of tospoviruses and animal-infecting bunyaviruses

The Bunyavirales: The Plant-Infecting Counterparts

Negative-strand (-) RNA viruses (NSVs) comprise a large and diverse group of viruses that are generally divided in those with non-segmented and those with segmented genomes. Whereas most NSVs infect animals and humans, the smaller group of the plant-infecting counterparts is expanding, with many causing devastating diseases worldwide, affecting a large number of major bulk and high-value food crops. In 2018, the taxonomy of segmented NSVs faced a major reorganization with the establishment of the order Bunyavirales. This article overviews the major plant viruses that are part of the order, i.e., orthospoviruses (Tospoviridae), tenuiviruses (Phenuiviridae), and emaraviruses (Fimoviridae), and provides updates on the more recent ongoing research. Features shared with the animal-infecting counterparts are mentioned, however, special attention is given to their adaptation to plant hosts and vector transmission, including intra/intercellular trafficking and viral counter defense to antiviral RNAi.

Recent Advances in Bunyavirus Glycoprotein Research: Precursor Processing, Receptor Binding and Structure

The Bunyavirales order accommodates related viruses (bunyaviruses) with segmented, linear, single-stranded, negative- or ambi-sense RNA genomes. Their glycoproteins form capsomeric projections or spikes on the virion surface and play a crucial role in virus entry, assembly, morphogenesis. Bunyavirus glycoproteins are encoded by a single RNA segment as a polyprotein precursor that is co- and post-translationally cleaved by host cell enzymes to yield two mature glycoproteins, Gn and Gc (or GP1 and GP2 in arenaviruses). These glycoproteins undergo extensive N-linked glycosylation and despite their cleavage, remain associated to the virion to form an integral transmembrane glycoprotein complex. This review summarizes recent advances in our understanding of the molecular biology of bunyavirus glycoproteins, including their processing, structure, and known interactions with host factors that facilitate cell entry.

Crystal structure of tomato spotted wilt virus GN reveals a dimer complex formation and evolutionary link to animal-infecting viruses

Tospoviridae is a family of enveloped RNA plant viruses that infect many field crops, inflicting a heavy global economic burden. These tripartite, single-stranded, negative-sense RNA viruses are transmitted from plant to plant by thrips as the insect vector. The medium (M) segment of the viral genome encodes two envelope glycoproteins, GN and GC, which together form the envelope spikes. GC is considered the virus fusogen, while the accompanying GN protein serves as an attachment protein that binds to a yet unknown receptor, mediating the virus acquisition by the thrips carrier. Here we present the crystal structure of glycoprotein N (GN) from the tomato spotted wilt virus (TSWV), a representative member of the Tospoviridae family. The structure suggests that GN is organized as dimers on TSWV's outer shell. Our structural data also suggest that this dimerization is required for maintaining GN structural integrity. Although the structure of the TSWV GN is different from other bunyavirus GN proteins, they all share similar domain connectivity that resembles glycoproteins from unrelated animal-infecting viruses, suggesting a common ancestor for these accompanying proteins.

Diterpenoid compounds from Wedelia trilobata induce resistance to Tomato spotted wilt virus via the JA signal pathway in tobacco plants

Tomato spotted wilt virus (TSWV) causes major losses of many crops worldwide. Several strategies have been attempted to control disease caused by TSWV. However, many challenges for the effective control of this disease remain. A promising approach is the use of abiotic or biotic inducers to enhance plant resistance to pathogens. We screened a diterpenoid compound from Wedelia trilobata, 3α-Angeloyloxy-9β-hydroxy-ent-kaur-16-en-19-oic acid (AHK), which had higher curative and protective effects against TSWV than the ningnanmycin control. The rapid initiation of the expression of all the TSWV genes was delayed by more than 1d in the curative assay, and the expression of the NSs, NSm and RdRp genes was inhibited. In addition, the replication of all TSWV genes in systemic leaves was inhibited in the protective assay, with an inhibition rate of more than 90%. The concentrations of jasmonic acid (JA) and jasmonic acid isoleucine (JA-ILE) in the AHK-treated and systemic leaves of the treated plants were significantly higher than those observed in the control. The results suggested that AHK can induce systemic resistance in treated plants. The transcription of the NtCOI1 gene, a key gene in the JA pathway, was significantly higher in both the inoculated and systemic leaves of the AHK-treated plants compared to the control. The AHK-induced resistance to TSWV in Nicotiana benthamiana could be eliminated by VIGS-mediated silencing of the NtCOI1 gene. These results indicated that AHK can activate the JA pathway and induce systemic resistance to TSWV infection.

Soybean vein necrosis virus: an emerging virus in North America

Few diseases have emerged in such a short period of time as soybean vein necrosis. The disease is present in all major producing areas in North America, affecting one of the major row field instead of row crops for the United States. Because of the significance of soybean in the agricultural economy and the widespread presence of the disease, the causal agent, soybean vein necrosis virus has been studied by several research groups. Research in the past 10 years has focused on virus epidemiology, management, and effects on yield and seed quality. This communication provides a review of the current knowledge on the virus and the disease.

Alstroemeria yellow spot virus (AYSV): a new orthotospovirus species within a growing Eurasian clade

An orthotospovirus distinct from all other orthotospoviruses was isolated from naturally infected alstroemeria plants. Disease symptoms caused by this virus mainly consisted of yellow spots on the leaves based on which the name alstroemeria yellow spot virus (AYSV) was coined. A host range analysis was performed and a polyclonal antiserum was produced against purified AYSV ribonucleoproteins which only reacted with the homologous antigen and not with any other (established or tentative) orthotospovirus from a selection of American and Asian species. Upon thrips transmission assays the virus was successfully transmitted by a population of Thrips tabaci. The entire nucleotide sequence of the M and S RNA segments was elucidated by a conventional cloning and sequencing strategy, and contained 4797 respectively 2734 nucleotides (nt). Simultaneously, a next generation sequencing (NGS) approach (RNAseq) was employed and generated contigs covering the entire viral tripartite RNA genome. In addition to the M and S RNA nucleotide sequences, the L RNA (8865 nt) was obtained. The nucleocapsid (N) gene encoded by the S RNA of this virus consisted of 819 nucleotides with a deduced N protein of 272 amino acids and by comparative sequence alignments to other established orthotospovirus species showed highest homology (69.5% identity) to the N protein of polygonum ringspot virus. The data altogether support the proposal of AYSV as a new orthotospovirus species within a growing clade of orthotospoviruses that seem to share the Middle East basin as a region of origin.

Iris yellow spot virus (Tospovirus: Bunyaviridae): from obscurity to research priority

TAXONOMY

Iris yellow spot virus (IYSV) is in the genus Tospovirus, family Bunyaviridae, with a single-stranded, tri-segmented RNA genome with an ambisense genome organization. Members of the other genera in the family infect predominantly vertebrates and insects.

GEOGRAPHICAL DISTRIBUTION

IYSV is present in most Allium-growing regions of the world.

PHYSICAL PROPERTIES

Virions are pleomorphic particles of 80-120 nm in size. The particle consists of RNA, protein, glycoprotein and lipids.

GENOME

IYSV shares the genomic features of other tospoviruses: a segmented RNA genome of three RNAs, referred to as large (L), medium (M) and small (S). The L RNA codes for the RNA-dependent RNA polymerase (RdRp) in negative sense. The M RNA uses an ambisense coding strategy and codes for the precursor for the GN /GC glycoprotein in the viral complementary (vc) sense and a non-structural protein (NSm) in the viral (v) sense. The S RNA also uses an ambisense coding strategy with the coat protein (N) in vc sense and a non-structural protein (NSs) in the v sense.

TRANSMISSION

The virus is transmitted by Thrips tabaci Lindeman (Order: Thysanoptera; Family: Thripidae; onion thrips) and with less efficiency by Frankliniella fusca Hinds (tobacco thrips). HOST: IYSV has a relatively broad host range, including cultivated and wild onions, garlic, chives, leeks and several ornamentals. Some weeds are naturally infected by IYSV and may serve as alternative hosts for the virus.

SYMPTOMS

IYSV symptoms in Allium spp. are yellow- to straw-coloured, diamond-shaped lesions on leaves and flowering scapes. Diamond-shaped lesions are particularly pronounced on scapes. As the disease progresses, the lesions coalesce, leading to lodging of the scapes. In seed crops, this could lead to a reduction in yield and quality. Early to mid-season infection in bulb crops results in reduced vigour and bulb size.

CONTROL

Resistant varieties are not available, but a limited number of accessions with field tolerance have been identified. Integrated disease management tactics, including sanitation, crop rotation, thrips management, maintenance of optimal plant vigour, soil fertility, irrigation and physical separation of bulb and seed crops, can mitigate the effect of the disease. Virus code: 00.011.0.85.009 Useful link: http://www.alliumnet.com/.

Sequence analysis of Indian iris yellow spot virus ambisense genome segments: evidence of interspecies RNA recombination

The nucleotide sequence of M- and S-RNA segments of an Indian iris yellow spot virus (IYSV) were determined. Sequence comparisons showed that both of these sequences shared less than 95 % identity with those other known IYSV isolates. Phylogenetic analysis revealed that the S- and M-RNA sequences of known IYSV isolates clustered with those of the tospoviruses, tomato yellow ring virus, polygonum ringspot virus and hippeastrum chlorotic ringspot virus. Further, multiple recombination detection methods detected inter- and intra-species recombination events that clustered primarily within the intergenic regions of S- and M-RNA, suggesting that these are possibly recombination hotspots in IYSV and closely related tospoviruses.

Global analysis of population structure, spatial and temporal dynamics of genetic diversity, and evolutionary lineages of Iris yellow spot virus (Tospovirus: Bunyaviridae)

Thrips-transmitted Iris yellow spot virus is an economically important viral pathogen of Allium crops worldwide. A global analysis of known IYSV nucleocapsid gene (N gene) sequences was carried out to determine the comparative population structure, spatial and temporal dynamics with reference to its genetic diversity and evolution. A total of 98 complete N gene sequences (including 8 sequences reported in this study) available in GenBank and reported from 23 countries were characterized by in-silico RFLP analysis. Based on RFLP, 94% of the isolates could be grouped into NL or BR types while the rest belonged to neither group. The relative proportion of NL and BR types was 46% and 48%, respectively. A temporal shift in the IYSV genotypes with a greater incremental incidence of IYSVBR was found over IYSVNL before 2005 compared to after 2005. The virus population had at least one evolutionarily significant recombination event, involving IYSVBR and IYSVNL. Codon substitution studies did not identify any significant differences among the genotypes of IYSV. However, N gene codons were minimally positively selected, moderately negatively selected denoting the action of purifying selection, thus rejecting the theory of neutral mutation in IYSV population. However, one codon position (139) was found to be positively selected in all the genotypes. Population selection statistics in the IYSVBR, IYSVNL genotypes and in the population as a whole also revealed the action of purifying selection or population expansion, whereas IYSVother displayed a decrease in population size. Genetic differentiation studies showed inherent differentiation and infrequent gene flow between IYSVBR and IYSVNL genotypes corroborating the geographical confinement of these genotypes. Taken together the study suggests that the observed diversity in IYSV population and temporal shift in IYSVBR genotype is attributable to genetic recombination, abundance of purifying selection, insignificant positive selection and population expansion. Restricted gene flow between the two major IYSV genotypes further emphasizes the role of genetic drift in modeling the population architecture, evolutionary lineage and epidemiology of IYSV.

Surface display of rice stripe virus NSvc2 and analysis of its membrane fusion activity

Rice stripe virus (RSV) infects rice and is transmitted in a propagative manner by the small brown planthopper. How RSV enters an insect cell to initiate the infection cycle is poorly understood. Sequence analysis revealed that the RSV NSvc2 protein was similar to the membrane glycoproteins of several members in the family Bunyaviridae and might induce cell membrane fusion. To conveniently study the membrane fusion activity of NSvc2, we constructed cell surface display vectors for expressing Nsvc2 on the insect cell surface as the membrane glycoproteins of the enveloped viruses. Our results showed that NSvc2 was successfully expressed and displayed on the surface of insect Sf9 cells. When induced by low pH, the membrane fusion was not observed in the cells that expressed NSvc2. Additionally, the membrane fusion was also not detected when co-expressing Nsvc2 and the viral capsid protein on insect cell surface. Thus, RSV NSvc2 is probably different from the phlebovirus counterparts, which could suggest different functions. RSV might enter insect cells other than by fusion with plasma or endosome membrane.

Molecular characterization of a new Tospovirus infecting soybean

A new, widespread disease was recently observed in soybean in the United States. The disease, named Soybean vein necrosis, is manifested by intraveinal chlorosis and necrosis, and has been found in almost all of the 50 fields visited over a period of 3 years in the midwest and midsouth part of the United States. A virus was isolated from symptomatic material, and detection protocols were developed. More than 150 symptomatic specimens collected from seven US States were tested, and all were found positive for the virus unlike 75 asymptomatic samples, revealing the absolute association between virus and disease. Protein pairwise comparisons coupled with phylogenetic analyses indicate that the virus is a new member of the genus Tospovirus.

Negative-strand RNA viruses: the plant-infecting counterparts

While a large number of negative-strand (-)RNA viruses infect animals and humans, a relative small number have plants as their primary host. Some of these have been classified within families together with animal/human infecting viruses due to similarities in particle morphology and genome organization, while others have just recently been/or are still classified in floating genera. In most cases, at least two striking differences can still be discerned between the animal/human-infecting viruses and their plant-infecting counterparts which for the latter relate to their adaptation to plants as hosts. The first one is the capacity to modify plasmodesmata to facilitate systemic spread of infectious viral entities throughout the plant host. The second one is the capacity to counteract RNA interference (RNAi, also referred to as RNA silencing), the innate antiviral defence system of plants and insects. In this review an overview will be presented on the negative-strand RNA plant viruses classified within the families Bunyaviridae, Rhabdoviridae, Ophioviridae and floating genera Tenuivirus and Varicosavirus. Genetic differences with the animal-infecting counterparts and their evolutionary descendants will be described in light of the above processes.

Structure and genome organization of the large RNA of iris yellow spot virus (genus Tospovirus, family Bunyaviridae)

The structure and organization of the large (L) RNA of iris yellow spot virus (IYSV) was determined, and with this report, the complete genomic sequence of IYSV of the genus Tospovirus, family Bunyaviridae has been elucidated. The L RNA of IYSV was 8,880 nucleotides in length and contained a single open reading frame in the viral complementary (vc) strand. The primary translation product of 331.17 kDa shared many of the features of the viral RNA-dependent RNA polymerase (RdRp) coded by L RNAs of known tospoviruses. The 5' and 3' termini of IYSV L RNA (vc) contain two untranslated regions of 33 and 226 nucleotides, respectively, and both termini have conserved terminal nucleotides, another common feature of tospovirus genomic RNAs. Conserved motifs characteristic of RdRps of members of the family Bunyaviridae were present in the IYSV RdRp.

Functional analysis of the Bunyamwera orthobunyavirus Gc glycoprotein

The virion glycoproteins Gn and Gc of Bunyamwera orthobunyavirus (family Bunyaviridae) are encoded by the M RNA genome segment and have roles in both viral attachment and membrane fusion. To investigate further the structure and function of the Gc protein in viral replication, we generated 12 mutants that contain truncations from the N terminus. The effects of these deletions were analysed with regard to Golgi targeting, low pH-dependent membrane fusion, infectious virus-like particle (VLP) formation and virus infectivity. Our results show that the N-terminal half (453 residues) of the Gc ectodomain (909 residues in total) is dispensable for Golgi trafficking and cell fusion. However, deletions in this region resulted in a significant reduction in VLP formation. Four mutant viruses that contained N-terminal deletions in their Gc proteins were rescued, and found to be attenuated to different degrees in BHK-21 cells. Taken together, our data indicate that the N-terminal half of the Gc ectodomain is dispensable for replication in cell culture, whereas the C-terminal half is required to mediate cell fusion. A model for the domain structure of the Gc ectodomain is proposed.

Nucleotide sequence and genome organization of the medium RNA of Iris yellow spot virus from the United States

Iris yellow spot tospovirus (IYSV) of the family Bunyaviridae causes a serious disease in onion in the USA and other parts of the world. Inspite of its economic importance, the complete genomic sequence of IYSV from the USA is not available. The genome structure and organization of the medium (M) RNA of a Washington (WA) isolate of IYSV were determined and compared to the corresponding region of two isolates previously described from Brazil and The Netherlands. Sequence analysis showed that the M-RNA was 4,817 nucleotides long and potentially coded for the movement protein (NSm) in the viral sense and the glycoprotein precursor (Gn and Gc) in the viral complementary sense. The predicted sizes of NSm and Gn/Gc precursor were 34.7 and 128.84 kDa, respectively. The two open reading frames are separated by a 380 nucleotide intergenic region. Phylogenetic analysis of the NSm and Gn/Gc genes from the WA isolate showed grouping that reflected their respective serogroups. The WA isolate formed a close cluster with the two previously reported IYSV isolates and the IYSV cluster was distinguishable from other tospovirus species. This is the first report of complete genomic sequence of the M-RNA of IYSV from the US.

Biological and molecular characterization of tospoviruses in Thailand

Twenty-eight isolates of tospoviruses associated with tomato, pepper, cucurbits, peanut, and Physalis plants collected from fields in different regions of Thailand were characterized. On the basis of N gene and protein sequence relationships, three tospoviruses were identified, namely Watermelon silver mottle virus (WSMoV), Capsicum chlorosis virus (CaCV), and Melon yellow spot virus (MYSV). CLUSTAL analysis of selected N protein sequences showed different isolates of CaCV in three distinct clades. Based on necrosis symptoms on tomato and their 93% identity to CaCV isolates in the other two clades, CaCV-TD8, CaCV-AIT and CaCV-KS16-Thailand tomato tospovirus were designated as CaCV-tomato necrosis strain. A phylogenetic tree based on the 413-amino-acid Gc fragment of the CaCV-Pkk isolate supported the existence of three distinct CaCV clades. Vigna unguiculata produced concentric rings useful for discriminating the Thai CaCV peanut isolates from tomato or pepper isolates. By using reverse transcription polymerase chain reaction with species-specific primers, the three tospoviruses could be detected in mixed infections in watermelon and Physalis, as well as in the bodies of thrips vectors, Thrips palmi and Scirtothrips dorsalis, collected from fields.

Insect vector interactions with persistently transmitted viruses

The majority of described plant viruses are transmitted by insects of the Hemipteroid assemblage that includes aphids, whiteflies, leafhoppers, planthoppers, and thrips. In this review we highlight progress made in research on vector interactions of the more than 200 plant viruses that are transmitted by hemipteroid insects beginning a few hours or days after acquisition and for up to the life of the insect, i.e., in a persistent-circulative or persistent-propagative mode. These plant viruses move through the insect vector, from the gut lumen into the hemolymph or other tissues and finally into the salivary glands, from which these viruses are introduced back into the plant host during insect feeding. The movement and/or replication of the viruses in the insect vectors require specific interactions between virus and vector components. Recent investigations have resulted in a better understanding of the replication sites and tissue tropism of several plant viruses that propagate in insect vectors. Furthermore, virus and insect proteins involved in overcoming transmission barriers in the vector have been identified for some virus-vector combinations.

The glycoprotein gene of Chrysanthemum stem necrosis virus and Zucchini lethal chlorosis virus and molecular relationship with other tospoviruses

Two tospoviruses, Chrysanthemum stem necrosis virus (CSNV) and Zucchini lethal chlorosis virus (ZLCV), cause economical losses in several ornamental and vegetable crops in Brazil. The nucleocapsid gene and movement protein sequences had already been reported for both viruses, though the glycoprotein precursor gene sequence was not available. In this study, cDNA fragments (ca. 4 kb) of the M RNA 3' portion of CSNV (isolate Chry-1) and ZLCV (isolate 1038), including the complete glycoprotein precursor gene, partial NSm gene, and the entire intergenic and 3' untranslated regions, were cloned and sequenced. The sequences were assembled with the corresponding 5' region sequence (NSm gene and 5'UTR) of the same isolates to build up the complete sequence of the M RNA segment of both species. The M RNA of CSNV was 4,828 nucleotide-long, while of ZLCV 4,836 nucleotides. Both M RNA molecules comprised two ORFs in an ambisense arrangement. The vcRNA coded for the viral glycoprotein (Gn/Gc) precursor gene of CSNV and ZLCV (both with 127.5 kDa). Comparison of deduced amino acids of the CSNV and ZLCV glycoprotein precursor genes with those of other tospoviruses showed the highest identity with that of Tomato spotted wilt virus (86%) and with that of CSNV (82%), respectively. However, the nucleotide sequence of the intergenic and 3' untranslated regions of CSNV and ZLCV shared lower identities with other tospoviruses. The glycoprotein precursor gene is thought to be a good candidate as additional classification parameter for Tospovirus taxonomy. The presence of the RGD motif in both Gc proteins indicated that they are typical American tospoviruses, which was confirmed by phylogenetic analysis. The membrane topology of both glycoproteins is discussed.

Tomato spotted wilt virus Gc and N proteins interact in vivo

Tomato spotted wilt virus (TSWV) virions consist of a nucleocapsid core surrounded by a membrane containing glycoproteins Gn and Gc. To unravel the protein interactions involved in the membrane acquisition of RNPs, TSWV nucleocapsid protein (N), Gn and Gc were expressed and analyzed in BHK21 cells. Upon coexpression of Gn, Gc and N, a partial colocalization of N with both glycoproteins was observed in the Golgi region. In contrast, upon coexpression of Gc and N in the absence of Gn, both proteins colocalized to a distinct non-Golgi perinuclear region. Using FLIM and FRET, interaction was demonstrated between N and Gc, but not between N and Gn, and was only observed in the region where both proteins accumulated. The genuine character of N-Gc interaction was confirmed by its presence in purified virus and RNP preparations. The results are discussed in view of TSWV particle assembly taking place at the Golgi complex.

The complete nucleotide sequence of a capsicum chlorosis virus isolate from Lycopersicum esculentum in Thailand

The complete nucleotide sequence of a tospovirus isolated from Lycopersicum esculentum in Thailand was determined. The L RNA comprises of 8912 nt and codes for the RNA-dependent RNA-polymerase (RdRp) (2877 aa). Two ORFs are located on the M RNA (4823 nt) encoding the non-structural (NSm) protein (308 aa) and the viral glycoprotein precursors (Gn/Gc) (1121 aa) separated by an intergenic region of 433 nt. ORFs coding for the non-structural (NSs) and nucleocapsid (N) protein, 439 aa and 275 aa, respectively, were identified on the S RNA (3477 nt) separated by an intergenic region of 1202 nt. The N protein of the Thailand isolate was most closely related to that of capsicum chlorosis virus (CaCV), sharing an amino acid sequence identity of 92.7%. Additionally, multiple sequence analyses revealed significant similarities to tospoviruses of the species Watermelon silver mottle virus and to several putative tospovirus entries in GenBank. Based on these alignments it is proposed to refer to all these different viruses as isolates of CaCV.

Hantavirus Gc glycoprotein: evidence for a class II fusion protein

Hantavirus cell entry is promoted by its envelope glycoproteins, Gn and Gc, through cell attachment and by fusion between viral and endosomal membranes at low pH. However, the role of Gn and Gc in receptor binding and cell fusion has not yet been defined. In this work, a sequence presenting characteristics similar to those of class II fusion peptides (FPs) of alphavirus E1 and flavivirus E proteins is identified within the hantavirus Gc glycoprotein. A three-dimensional comparative molecular model based on crystallographic data of tick-borne encephalitis virus E protein is proposed for the Andes virus (ANDV) Gc ectodomain, which supports a feasible class II fusion-protein fold. In vitro experimental evidence is provided for the binding activity of the ANDV FP candidate to artificial membranes, as demonstrated by fluorescence anisotropy assays. Taken together, these results support the hypothesis that the Gc glycoprotein of hantaviruses and of other members of the family Bunyaviridae directs the viral fusion activity and that it may be classified as a class II viral fusion protein.

Tomato spotted wilt virus glycoprotein G(C) is cleaved at acidic pH

Tomato spotted wilt virus (TSWV) is a plant-infecting member of the family Bunyaviridae. TSWV encodes two envelope glycoproteins, G(N) and G(C), which are required for virus infection of the arthropod vector. Other members of the Bunyaviridae enter host cells by pH-dependent endocytosis. During this process, the glycoproteins are exposed to conditions of acidic pH within endocytic vesicles causing the G(C) protein to change conformation. This conformational change renders G(C) more sensitive to protease cleavage. We subjected TSWV virions to varying pH conditions and determined that TSWV G(C), but not G(N), was cleaved under acidic pH conditions, and that this phenomenon did not occur at neutral or alkaline pH. This data provides evidence that G(C) changes conformation at low pH which results in altered protease sensitivity. Furthermore, sequence analysis of G(C) predicts the presence of internal hydrophobic domains, regions that are characteristic of fusion proteins. Like studies with other members of the Bunyaviridae, this study is the first step towards characterizing the nature of cell entry by TSWV.

Nucleotide sequence of melon yellow spot virus M RNA segment and characterization of non-viral sequences in subgenomic RNA

The nucleotide sequence of melon yellow spot virus (MYSV) M RNA segment was determined. The M RNA segment contains one open reading frame (ORF) encoding 308 amino acids (aa) in the sense orientation and another ORF encoding 1,127 aa in the complementary orientation, which were homologous to the NSm protein and G1/G2 glycoprotein precursor (Gp) protein, respectively. Amino acid sequences identities with the other tospovirus suggested that MYSV is closely related to groundnut bud necrosis virus and watermelon silver mottle virus. To analyze subgenomic RNA of the M RNA segment, RNA transcripts corresponding to the NSm and Gp genes were specifically amplified, and the nucleotide sequence of the 5' terminal region was determined. Sequence analysis of the NSm and Gp transcripts showed that they had a non-viral sequence 12-18 and 10-18 nucleotides long, respectively. Although these sequences varied considerably, in more than half of the cases, a cytosine residue was observed at the 3' end of the non-viral leader sequence, which suggests that the viral transcriptase prefers certain cap-donor sequences harboring a 3'CA dinucleotide.

Sequence analysis of the glycoproteins of Tomato chlorotic spot virus and Groundnut ringspot virus and comparison with other tospoviruses

The tospoviruses Tomato chlorotic spot virus (TCSV) and Groundnut ringspot virus (GRSV) cause high economic losses in several vegetable crops in Brazil. The glycoprotein precursor coding sequence was still not available for these two viruses. In this study, the 3' 4 kb M RNA of TCSV and GRSV genome was cloned and sequenced. The sequences were compiled with the available 5' region sequence (NSM gene and 5' UTR) of the same isolates. The M RNA of TCSV was deduced as formed by 4,882 nucleotides, while of GRSV by 4,855 nucleotides. Both M RNA comprised two ORFs in an ambisense arrangement. The vcRNA ORF coded for viral glycoprotein (G1/G2) precursor of TCSV (128.46 kDa) and for glycoprotein precursor of GRSV (128.16 kDa). Comparison of the TCSV and GRSV glycoprotein precursor proteins with those of other tospoviruses showed the highest identity with Tomato spotted wilt virus (81 and 79%, respectively). The amino acid sequence comparison of glycoprotein precursor between TCSV and GRSV revealed a high identity of 92%. However, the nucleotide sequence of the M RNA intergenie region showed only 78%. Phylogenetic analysis was done based on glycoprotein precursor and on M RNA intergenic region of tospoviruses and parameters on tospovirus taxonomic classification were discussed.

Tospovirus-thrips interactions

The complex and specific interplay between thrips, tospoviruses, and their shared plant hosts leads to outbreaks of crop disease epidemics of economic and social importance. The precise details of the processes underpinning the vector-virus-host interaction and their coordinated evolution increase our understanding of the general principles underlying pathogen transmission by insects, which in turn can be exploited to develop sustainable strategies for controlling the spread of the virus through plant populations. In this review, we focus primarily on recent progress toward understanding the biological processes and molecular interactions involved in the acquisition and transmission of Tospoviruses by their thrips vectors.

Proteomics computational analyses suggest that the carboxyl terminal glycoproteins of Bunyaviruses are class II viral fusion protein (beta-penetrenes)

The Bunyaviridae family of enveloped RNA viruses includes five genuses, orthobunyaviruses, hantaviruses, phleboviruses, nairoviruses and tospoviruses. It has not been determined which Bunyavirus protein mediates virion:cell membrane fusion. Class II viral fusion proteins (beta-penetrenes), encoded by members of the Alphaviridae and Flaviviridae, are comprised of three antiparallel beta sheet domains with an internal fusion peptide located at the end of domain II. Proteomics computational analyses indicate that the carboxyl terminal glycoprotein (Gc) encoded by Sandfly fever virus (SAN), a phlebovirus, has a significant amino acid sequence similarity with envelope protein 1 (E1), the class II fusion protein of Sindbis virus (SIN), an Alphavirus. Similar sequences and common structural/functional motifs, including domains with a high propensity to interface with bilayer membranes, are located collinearly in SAN Gc and SIN E1. Gc encoded by members of each Bunyavirus genus share several sequence and structural motifs. These results suggest that Gc of Bunyaviridae, and similar proteins of Tenuiviruses and a group of Caenorhabditis elegans retroviruses, are class II viral fusion proteins. Comparisons of divergent viral fusion proteins can reveal features essential for virion:cell fusion, and suggest drug and vaccine strategies.

The two envelope membrane glycoproteins of Tomato spotted wilt virus show differences in lectin-binding properties and sensitivities to glycosidases

Tomato spotted wilt virus (TSWV, Genus: Tospovirus, Family: Bunyaviridae) is a major constraint to the production of several different crops of agronomic and horticultural importance worldwide. The amino acid sequence of the two envelope membrane glycoproteins, designated as G(N) (N-terminal) and G(C) (C-terminal), of TSWV contain several tripeptide sequences, Asn-Xaa-Ser/Thr, suggesting that the proteins are N-glycosylated. In this study, the lectin-binding properties of the viral glycoproteins and their sensitivities to glycosidases were examined to obtain information on the nature of potential oligosaccharide moieties present on G(N) and G(C). The viral proteins were separated by sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) and probed by affinoblotting using a battery of biotinylated lectins with specificity to different oligosaccharide structures. G(C) showed strong binding with five mannose-binding lectins, four N-acetyllactosamine-binding lectins and one fucose-binding lectin. G(N) was resolved into two molecular masses and only the slow migrating form showed binding, albeit to a lesser extent than G(C), with three of the five mannose-binding lectins. The N-acetyllactosamine- and fucose-specific lectins did not bind to either molecular mass form of G(N). None of the galactose-, N-acetylgalactosamine-, or sialic acid-binding lectins tested showed binding specificity to G(C) or G(N). Treatment of the denatured virions with endoglycosidase H and peptide:N-glycosidase F (PNGase F) resulted in a significant decrease in the binding of G(C) to high mannose- and N-acetyllactosamine-specific lectins. However, no such differences in lectin binding were apparent with G(N). These results indicate the presence of N-linked oligosaccharides of high mannose- and complex-type on G(C) and possibly high mannose-type on G(N). Differences in the extent of binding of the two envelope glycoproteins to different lectins suggest that G(C) is likely to be more heavily N-glycosylated than G(N). No evidence was observed for the presence of O-linked oligosaccharides on G(N) or G(C).